If you just want some low greenery, you can try anubias "nana petite" or, like SnakeIce suggested, moss; both tied to rocks. Pea gravel is hard to plant IME, it's so big & heavy. Crypts can grow in it but roots may be damaged at first (crypt leaves often "melt" when new but don't worry, they should regrow if roots are healthy). In low ferts, low-med light, my wendtii are fairly low, 3-5 inches, & some are red(ish) or maroon, a nice contrast, I love crypts!
If you do switch Eco, you might try glossostigma, or again as SI said, marislea. But both are pretty slow growing without regular ferts & moderate light. Dwarf sag seems to grow too big if in the foreground or too short if mid IME. Don't expect a " full dense carpet" whichever you choose in a low tech tank. I go for a nice clump in a few areas at most & I am happy with that.
If you want a large bushy fine leaved plant for the back right, I'd suggest watersprite. It's easy in lowish light but it's a fern, so while it's roots can be planted in gravel the "crown" (where roots & leaves join) needs to be above the substrate a tiny bit. I love mine in a 75g but not for a much smaller tank, it's BIG!
You have a nice start... but it seems like either your stems are struggling or just really new. Might be slightly crushed at the bottom (again, pea gravel is hard to plant) or a little starved for ferts &/or light. If it continues to looks sparse at the bottom even after you trim & replant the "good" tops you may want to adjust them, your ferts or their placement so they get the best light available.
Depending on placement the various mosses can be used in either foreground or midground. If used in foreground you can tie them to a small flat stone so you can pull that out and trim as needed to keep the look.
